A bottom bracket assembly of a bicycle includes a shaft rotatably supported in a housing box suitably provided in the bicycle frame, and two crank arms associated with the opposite ends of the shaft. The shaft can be formed as a distinct piece from the crank arms, or it can be formed as a single piece with one of the two crank arms. Alternatively, the shaft can be formed as two or more pieces connected together through suitable connection means, each piece being formed integral with or distinct from a respective crank arm.
For the sake of simplicity of explanation, throughout the present description and subsequent claims, the embodiment in which the shaft is formed as a single piece will typically be described, except when expressly indicated the contrary. It should in any case be understood that what has been described is also applicable to a shaft which is formed as a plurality of pieces.
Moreover, throughout the present description, the component of the bottom bracket assembly of a bicycle consisting of the shaft (or of a shaft element), and of a crank arm, mutually coupled or formed as a single piece, will be identified as the “crank arm assembly.”
The rotation of the shaft of the bottom bracket assembly with respect to the housing box is achieved through the use of a pair of rolling bearings mounted on the shaft. In particular, each bearing is typically mounted on the shaft at a respective end portion of the shaft body, in a position adjacent to the crank arm. When the bottom bracket assembly is mounted on the bicycle frame, each bearing is thus operatively interposed between the shaft and the respective housing box provided in the bicycle frame.
The bearings are typically supported outside of the aforementioned box through suitable adaptation elements which are mounted at the opposite free ends of the shaft-housing box.
Typically, the dimensions of the housing box of the shaft of the bottom bracket assembly and the dimensions of the individual components of the bottom bracket assembly, i.e. of the bearings, of the possible adaptation elements and of the shaft, are selected so as to ensure a desired rigidity in terms, for example, of resistance to the bending moment and torsional moment acting upon the shaft of the bottom bracket assembly due to the action imparted by the cyclist during pedaling.
For this purpose, the housing box of the shaft of the bottom bracket assembly typically consists of a hollow cylinder of standard length equal to 68 mm. Such a cylinder is internally threaded at its opposite free ends to receive, by screwing, a corresponding threaded portion of the respective adaptation element. Standardized threading M36×24 tpi (teeth per inch) or BC-1.37″×24 tpi is typically used, made on a diameter of the cylinder of about 34 mm. The shaft used is typically made from steel and has an outer diameter of 25 mm. Each bearing has an inner diameter of 25 mm and an outer diameter of 37 mm, whereas the distance between the median planes of the two bearings, along the longitudinal direction of the shaft, is equal to about 85 mm.